Ceramic honeycomb structures are actively used in various applications such as a catalyst carrier applied to combustion equipment such as a boiler and the like, internal combustion engines such as a diesel engine and the like, a chemical reactor, a reformer for fuel cell and the like, or a filter for trapping and removing particulate matters contained in exhaust gases, by making use of the characteristics of ceramic quality excellent in heat resistance.
Although these honeycomb structures are installed in a case with the outer peripheral side thereof gripped, they are often installed in such a manner that the outer peripheral side thereof is gripped with a large force as in a case that they are installed in the vicinity of an engine which is continuously subjected to large vibrations. Accordingly, it is very important in practical use that they have large compression strength on the outer peripheral side thereof so that they are not broken even if they are installed in the case while being gripped with a relatively large force.
Further, in a honeycomb structure used as, for example, an exhaust gas purifying filter, a filter regenerating process is executed therein to combust and remove carbon fine particles accumulated therein. In the filter regenerating process, the honeycomb structure is locally exposed to a high temperature. At the time, a problem arises in that cracks and the like are liable to occur in the honeycomb structure in a relatively short time of use because the thermal stress of the honeycomb structure is locally increased due to a difference of thermal expansion among respective portions thereof.
Further, a honeycomb structure used as a catalyst, a filter and the like for purifying exhaust gases is required to further improve a purifying capability by more reducing a thermal capacity and a pressure loss, improving a trapping efficiency, and the like. Thus, a decrease in thickness of the partition walls of the honeycomb structure and an increase in the porosity thereof are in progress in correspondence to the above requirement.
Incidentally, there has been proposed a honeycomb structure that is made by joining a plurality of honeycomb segments in order to be formed in a desired shape through joint members as a countermeasure against the problem of occurrence of cracks and the like in the honeycomb structure which are caused by being locally exsposed to high temperature (refer to Japanese Patent Publication (KOKOKU) No. 61-51240 and Japanese Patent Application Laid-Open No. 8-28246).
Further, in a honeycomb structure made integrally in its entirety, there has been proposed a structure, in which the partition walls of an outer peripheral portion are made thicker than internal partition walls, or the porosity of the partition walls thereof is reduced to provide sufficient compression strength even when the outer periphery of the honeycomb structure is gripped with a large force (refer to Japanese Patent Application Laid-Open Nos. 54-150406 and 55-147154).
However, any countermeasure is not taken into consideration to prevent breakages which selectively occur in particular portions of the so-called segment type honeycomb structure composed of a plurality of segments joined to each other when the honeycomb structure is used in a case by being gripped therein. Accordingly, when the honeycomb structure is mounted, a sufficient performance cannot be necessarily obtained. Further, in the honeycomb structure made integrally as a whole, the entire partition walls of the outer peripheral portion thereof are made thicker than inner partition walls and the porosity and the like thereof are reduced. With this arrangement, a thermal capacity and a pressure loss are greatly increased or a trapping efficiency is greatly reduced. Accordingly, a high purification performance, which is required by the times, cannot be achieved while satisfying desired compression strength.
Further, in the honeycomb structure made integrally as a whole, when a honeycomb structure in which partition walls are thicker in an outer peripheral portion than in an inner portion is made, and when a honeycomb structure in which cell density is larger in an outer peripheral portion than an inner portion is made, a difference of extrusion resistance is increased among the respective portions of the honeycomb structure by greatly changing a partition wall thickness and a cell density, from which a problem in extrusion forming arises in that defective forming is liable to occur in a resultant honeycomb structure. Likewise, when a honeycomb structure and the like, in which the porosity of the partition walls of an outer peripheral portion is smaller than that of the partition walls of an inner portion, is made, a material used in the partition walls of the outer peripheral portion is ordinarily replaced by a material having a smaller porosity. In the method, however, it is very difficult to provide the porosity of the partition walls with a desired change.